Conventional fuel systems for vehicles with internal combustion engines may include a canister that accumulates fuel vapor from the headspace of the fuel tank. The canister typically contains an adsorption medium such as activated charcoal that adsorbs hydrocarbon pollutants in the vented fuel vapor before the vent effluent is released into the atmosphere.
Activated charcoal removes organic pollutants by adsorption, a process whereby the pollutants are attracted to the relatively large surface area of the charcoal particles. The charcoal becomes saturated over time, however, and the canister must be purged to remove the pollutants so that more may be adsorbed.
In a typical automotive fuel system, the charcoal canister is purged by using intake manifold vacuum to draw outside air through the canister. The volatile organic compounds that are purged from the adsorption medium in the canister are transferred to the engine combustion chambers for combustion.
Hydrocarbon pollutants accumulate in the charcoal canister during periods when the engine is off and the vehicle is not in use. During those periods, it is not possible to purge the canister because no manifold vacuum is available. It is therefore important that the canister be purged to the greatest extent possible during those times when the engine is running, and manifold vacuum is available.
Various government regulatory agencies, such as the U.S. Environmental Protection Agency and the Air Resources Board of the California Environmental Protection Agency, have promulgated standards related to limiting fuel vapor released into the atmosphere. To comply with those standards, the adsorption canister must be purged regularly to free it from accumulated pollutants. In that way, it is assured that hydrocarbons are efficiently removed fuel tank vapors vent to atmosphere.
In a typical fuel tank ventilation system 100, shown in FIG. 1, vapors from a fuel tank 110 are passed through an adsorption canister 120 containing activated charcoal, and are vented 130 to atmosphere. A dust filter (not shown) is typically used on the vent 130 to prevent particulate contaminants from entering the system. Vapors are caused to flow from the fuel tank out the vent by natural pressure in the tank caused by temperature increases and volatility of the fuel.
A canister purge valve 140 is opened to purge the adsorption canister 120 with outside air from the vent 130. When an engine control unit (ECU) 145 determines that the canister should be purged, the ECU opens the purge valve 140, applying vacuum from the intake manifold 150 to the canister 120. Outside air is drawn from the vent 130 through the charcoal medium in the canister 120, purging the charcoal of accumulated hydrocarbons. The gaseous mixture passes through the valve 140, through the intake manifold 150 and into the engine 160, where the purged hydrocarbons combust with fuel from a fuel injection system (not shown). The ECU 145 may regulate the opening of the valve 140 to accommodate various engine conditions, ambient outside air conditions and other factors. The ECU may receive information from sensors such as an exhaust gas oxygen sensor (not shown) and regulate the purge valve to maintain stoichiometric proportions in the engine combustion chambers.
For the ECU to control canister purge flow without extensive custom programming, the canister purge valve must have a reasonably linear response over its duty cycle. For example, flow should start upon application of a threshold operating current, and flow should increase approximately linearly with the application of increasing operating current. The approximately linear operating characteristics should be maintained over a range of pressure differentials across the valve, so that there is a predictable purge flow response over the expected range of intake manifold vacuum pressures.
Certain engine designs have inherently low intake manifold vacuum. Those engine designs include hybrid engines, multidisplacement engines and direct injection engines. In each of those cases, the low manifold vacuum, combined with the small flow path diameters typical of a canister purge valve as described above, provides insufficient purge flow to clean the canister.
Several solutions have been tried to solve the problem of insufficient canister purge resulting from low intake manifold vacuum. Some prior systems include multiple purge valves, or a large, heavy purge valve with increased port and sealing diameters. Those systems are more expensive to manufacture, and are difficult to validate in production. Such systems must be custom designed for each application, and cannot be easily added to an existing canister purge system design, further increasing manufacturing and development costs.
There is therefore presently a need for a method and system for providing a sufficient purge flow to remove adsorbed hydrocarbons from an adsorption canister in a vehicle fuel tank ventilation system, in cases where only low vacuum is available from the engine intake manifold. To the inventor's knowledge, no such method and system are currently available.